US accusations of a covert programme led to the breakdown of a 1994 deal to freeze the North's plutonium-based nuclear programme, and last October the reclusive regime staged its first nuclear test.

But suspicions were aired earlier this year about the quality of the US intelligence which prompted the claims.

"They were (pursuing a HEU programme). I definitely thought they were. I don't think that those facts are in question," former US nuclear envoy on North Korea James Kelly told AFP.

"It was crystal-clear that they invested hundreds of millions of dollars in it over quite a few years, going back into certainly the late 80s and possibly the early 90s," the former assistant secretary of state said of the programme.

Speaking on the sidelines of a Seoul forum, he said he could not be certain the North is still pursuing the programme as he no longer has access to intelligence.

Kelly in 2002 met North Korean first vice foreign minister Kang Sok-Ju, who according to US officials admitted the existence of the covert HEU programme.

"All of the US officials dependently and independently understood he (Kang) acknowledged the existence of the HEU programme," David Straub, who accompanied Kelly on the 2002 Pyongyang trip, told the forum.

Pyongyang denied having a HEU programme, and in protest at the accusations reactivated its plutonium-producing nuclear reactor, expelled UN atomic inspectors and eventually tested its first atomic bomb.

In February it agreed on a deal to scrap all its nuclear programmes in exchange for aid and diplomatic benefits. But the first phase of disarmament is being delayed by a banking row.

Kelly's successor Christopher Hill said in March that North Korea should come clean about the HEU programme as well as its plutonium-based one.

US intelligence agencies came in for harsh criticism after President George W. Bush's administration in February seemed to scale back the certainty it had expressed in 2002 about the HEU programme.

"We still have confidence that the programme is in existence -- at the mid-confidence level," Joseph De Trani, North Korea mission manager for the US Director of National Intelligence, told the US Senate Armed Services Committee then.

De Trani later said intelligence agencies "have at least moderate confidence that North Korea's past efforts to acquire a uranium enrichment capability continue today."

Kelly said in a report to the forum that North Korea is still believed to harbour nuclear ambitions despite the disarmament deal.

"It is unlikely, yet at least, that North Korea has made the strategic choice to give up its nuclear weapons," he said in the report.

He warned South Koreans against seeing the nuclear problem as more of a US-North Korean bilateral issue than one involving themselves.

"There is a tendency to see North Korea's nuclear weapons, not as a threat to South Korea, but as some dispute that only involves North Korea and the United States. Such an attitude is a serious mistake."

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